@mattdm yes and no - no stackexchange site can change the Flickr file size restrictions (15 MB for free, 20 MB for pro (source)). I think the real meat of this question is how to reduce the file size of the photos from >15 to <15.
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koiyuMay 4 '11 at 19:48

Well, the meat of the question is that now, after you've tastefully formed it into a hamburger with your edits. Not that there's anything wrong with that. :)
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mattdmMay 5 '11 at 1:57

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Note Flickr now allows uploading photos of up to 200MB (with a free account).
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vclawMar 16 at 12:24

I know you asked about Photoshop, therefore I'll post this as a comment, not an answer. In Lightroom, it has this beautiful export setting where you set the maximum file size, after which LR will simply figure out itself which quality settings to use.
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FerdyMay 19 at 20:14

8 Answers
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If you are saving to JPEG after processing check your compression settings. File size can climb astonishingly high the closer you get to 100% quality without any noticeable difference in quality. Dropping it down to around 90% can cut file size quite a bit.

What format are you saving the files in? If you're saving as TIFF or PSD, you could instead try saving to JPEG, which will be much smaller. If you're using JPEG and the files are still over 15 MB, those must be some HUGE images.

(Flickr is probably just going to convert the file to JPEG anyways to save on bandwidth when serving the images, so it's not like you're going to be able to get away with a loss-less format.)

There are two settings that affect JPEG file size, resolution and compression. Experiment with both these settings until you get a balance between the two that works for you. All other things being equal, I usually compress my images more rather than resizing them more, however Flickr has size limits that may affect your choice here. Always remember to keep an original uncompressed and unresized copy of your photo as well!

Compression

When you save an image as a JPEG, the quality of the image is degraded in order to make the file smaller. You can control the filesize versus quality tradeoff in photoshop when selecting File and Save as... and choosing the JPEG format. You will be prompted for a compression setting. I never set this to more than 10 and neither should you, and settings down as low as 8 are generally satisfactory for me. When you set the JPEG quality, photoshop will estimate what the resulting filesize will be, enabling you to tailor this setting for the desired file size:

Resolution

Resolution is the size of your image in pixels (or megapixels). My 550D saves images at 5184 x 3456 pixels, which is almost always way more than I need. For the purpose of posting to Flickr, unless you have a Pro account the practical maximum dimensions is 1024 pixels along the longest size, so there is little point in having a larger image than that. You can reduce filesize by resizing your image to a smaller size by going to the Image menu and select Image size:

You have already gotten plenty of good answers. I do wish to add that there should be a separation between the size of your images, the size of your in working files and the size of the images you share:

Size of your images: That is the size of the output of your camera. Usually 5-10 MB for JPEGs and 10-24 MB for RAW files. This is the amount of data your camera has captured. It is highly recommended these never be overwritten.

Size of your working files: This the files you work in Photoshop (or similar traditional image editing applications). These files get very big because they contain layer information that allows you to modify some of the work you have already done. While these are larger than your originals, the increase size does not reflect more details.

Size of shared images: This is the size of the images that you feel comfortable sharing, potentially limited by the platform (Flick, Picasa, etc) and your plan.

In your case, what you want is to Export a version of your images for upload to Flickr. There are a number of tools to do that for you. Even Adobe's Lightroom will let you do that directly without you managing the intermediate files, it only asks what size you want to share them at. Picasa too can do this if you add the picasa2flickr plugin.

I suggest you resize the photos in Photoshop, go to the image menu, select "Image Size..." in the document size box enter something like 25%, save the image and you should be able to upload them onto Flickr without problems.

Don't just resize by 25% as you may lose resolution! Rather, resize it to the maximum Flickr allowable size of 1024 pixels on the long side. And, as @Henry Jackson mentioned, save it as JPEG.
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ysapMay 4 '11 at 20:03

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@ysap Flickr does not have a maximum dimension limit. Although only pro accounts can access full-size images, there is no limit to the dimension of images uploaded.
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ahockleyMay 4 '11 at 20:08

@ahockley - for free account, the images are resized to at-most the max res and you don't have access to the source. Effectively, you can have the same result resizing yourself before uploading.
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ysapMay 4 '11 at 20:10

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@ysap though if one desires to upgrade to the Pro account later, the originals would be in their full (original) size without the need of a re-upload. But if someone is quite sure he/she isn't going to upgrade or if the uploaded collection is small, I agree with you.
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koiyuMay 4 '11 at 20:48

A quick tip: Photoshop files are .psd files which have extra Photoshop editing information. To publish or share with others in Flickr convert that file from psd format to jpg, png or gif as per requirement. Generally jpg is more suitable.

Now for reducing size:
1.) Check image size in Photoshop using image->image size and set it to nearly 800x600 or something that you think more appropriate for sharing. Remember the larger the dimension larger the file size.
2.) Save it with less jpg quality. Again larger the quality larger the size. It can be done using File->Save for web menu.